Stratigraphy is fundamental to the discipline of geology in describing the spatial, geometrical, structural, sequential and temporal relationships of rock units. In response to the formation of rocks in highly variable depositional environments and with varying sedimentary compositions, stratigraphic approaches span a wide range of disciplines, such as, litho-, bio-, chrono-, magneto-, seismic-, sequence- and chemo-stratigraphy.
Generally, in early stage geological exploration, little or no information is available on sediment characteristics. The identification and analysis of a potential hydrocarbon reservoir is a matter of interpretation and analysis of seismic reflection data.
Seismic surveying is generally performed by imparting energy to the earth at one or more source locations, for example, by way of controlled explosion, mechanical input etc. Return energy is then measured at surface receiver locations at varying distances and azimuths from the source location. The travel time of energy from source to receiver, via reflections and refractions from interfaces of subsurface strata, indicates the depth and orientation of such strata.
Seismic images indirectly depict the distribution of material deposited over large areas. Exploiting the fact that different seismic signatures characterize different depositional settings, seismic sequence stratigraphy can be used to infer depositional settings and their spatial and temporal distribution.
Deciphering geological history in this way enables analysis and estimation of probable sedimentary characteristics. However, performing sequence stratigraphic analysis over large seismic volumes is a daunting task, particularly if done manually. Hence, there is a need for automated analysis tools.
US 2004/0260476 discloses a method for automated extraction of surface primitives from seismic data.
For example, one embodiment of the method of US 2004/0260476 involves defining, typically with sub-sample precision, positions of seismic horizons through an extrema representation of a 3D seismic input volume; deriving coefficients that represent the shape of the seismic waveform in the vicinity of the extrema positions; sorting the extrema positions into groups that have similar waveform shapes by applying classification techniques with the coefficients as input attributes using unsupervised or supervised classification based on an underlying statistical class model; and extracting surface primitives as surface segments that are both spatially continuous along the extrema of the seismic volume and continuous in class index in the classification volume.
Subsurface stratification results from the nature of deposition—newer sediments being deposited on top of older sediments. The spatial (and temporal) variability of stacking patterns, or sequences, observed in seismic images relates to depositional environments and post-depositional processes, such as erosion and tectonic activity.
Linking reflection patterns to depositional environments, and vertical stacking order to order of deposition and, thus, relative timing, enables the geological history of the subsurface to be deciphered.
Because stacking order relates to relative time of deposition, it can be possible to show graphically the depositional history of observed seismic data. For example, flattening the sedimentary layers and plotting them according to their time of deposition provides a useful mapping known as a chronostratigraphic chart. This is a graphic display, with geologic age along the vertical axis and distance along the horizontal axis, providing a visualisation of the relative ages and geographic extent of sediments over a given area. The time of deposition of each sediment is generally relative, unless extra information e.g. from well logs is available whereby exact dating may be performed.
U.S. Pat. No. 6,708,118 proposes a system for utilizing a geological age volume to investigate a portion of the earth in which a geologic age is selected, a search being made in the geological age volume for locations having substantially the selected geological age. U.S. Pat. No. 6,850,845 and U.S. Pat. No. 6,853,922 propose a related system and method.